Contemporary computer operating systems usually support disk snapshot acquisition technology (e.g. Microsoft provides a service called Volume Shadow Copy Service (VSS) in Windows XP and Windows Server 2003). A snapshot technology is a file system preserving mechanism. A disk snapshot differs from a regular data backup duplicating in that it does a fast system file backup of a disk volume (a disk volume consists of disk chunks with data stored in them) at a random point in time and no file duplication is involved when a snapshot is being acquired. Directory structures in the data structure of the file system in use are therefore preserved. Different versions of snapshot can be promptly established for a disk volume at various point of time to compensate for the lack of timeliness in traditional backup mechanism. In addition, when accidents such as a virus attack or disk breakdown happens to a disk drive, the administrator can restore a pre-accident snapshot to replace a disk volume and minimize damage.
As aforementioned, no file duplication is involved when a disk snapshot is taking place. Instead, when there is any change in the disk chunks of a disk volume, first original data in said disk chunks are duplicated to other disk chunks not belonged to said volume in the hard disk, original data in said disk chunks of said disk volume are then modified as intended. Furthermore, in a kernel memory space provided by the operating system (e.g. Linux) in a server, a Copy-On-Write (COW) table is established to record information of the destination disk chunk for a disk chunk storing original data. Said technology is called Copy-On-Write technology.
However, an operating possesses a limited kernel space and the kernel space does not grow with an increase in system memory space. The COW table occupies more space in the kernel space when more disk chunks of a disk volume require modifications. After taking a significant number of snapshots of a disk volume at different point of time, or a rapid increase in disk volume size, the kernel space will be completely occupied by a great amount of COW tables and finally exhausted, causing a breakdown in the operating system. Consequently, development of a disk snapshot acquisition method to solve the above-mentioned problems has become a critical issue.